Extraction tube for extracting lower molecular components from granulated polyamides



Jan. 21, 1969 .1. GOETZKE 3,423,183

EXTRACTION TUBE FOR EXTRACTING LOWER MOLECULAR COMPONENTS FROM GRANULATED POLYAMIDES Filed Feb. 16, 1967 VIBRATING CHUTE OUTLET 5-WATER INLET I souos 8 2 OUTLET IN VEN TOR. Jl/ERGf/V 60572/(5 gm y, %wzmam 9 United States Patent US. Cl. 23-270 Int. Cl. B01j 9/06; B01d 11/02; B01d /02 ABSTRACT OF THE DISCLOSURE An upright extraction tube which removes lower molecular components from granulated polyamides by the use of solvents, has a plurality of restriction means in the tube. The granulated polyamides are introduced at the upper end of the tube while the solvents are introduced at the lower end of the tube and the narrowest portion of the restriction means causes the upward flow of the solvent at that point to be at least equal to counterflow currents of the solvent.

BACKGROUND OF THE INVENTION Field of the invention and description of the prior art This invention relates to an extraction tube for continuously removing lower molecular materials from granulated polyamides.

It is well known that polyamides, particularly those of the type of poly-e-caprolactam, contain after completion of polymerization considerable amounts (up to 12% by weight) of the starting monomers as Well as small amounts of oligomers. The oligomers are troublesome since the mechanical properties of the products of polymerization, particularly nylon threads and fibers, are severely impaired. It is therefore common to reduce the content of lower molecular components by extraction with solvents, preferably water, to lower concentrations of, for example, about 0.5% by weight.

It has been the usual practice to perform such extractions discontinuously in large washing boilers or continuously in extraction towers. The discontinuous extraction processes have the known drawbacks of down time on opening, closing, and charging the extraction installation, and low efficiency due to waste of input energy because of periodic reheating for each batch.

In usual continuous extraction processes, the granulate is introduced into an extraction tube at the top and hot water enters at the bottom thereof and flows upwardly counterflow to the polyamide particles. During the downward movement of the particles in the water stream, the extraction of the lower molecular components occurs. At the bottom of the extraction tube the granulate is discharged, while the water which has become charged with lower molecular components is drawn off at the upper part of the extraction tube. Inside the elongated extraction tube, counterflow currents are set up because of density fluctuations in the water. The density fluctuations occur for various reasons. Sometimes the density varies because of excessive heating, for example, along the heated wall. It also increases because of cooling in the upper part of the tube upon entry of the considerably colder polyamide cuttings. These temperature variations result in corresponding convection currents. Upon charging of the water with the heavier lower molecular components, especially monomers, the density increases in "ice the main flow direction of the water, which is an additional cause for counterflow currents. The total result of the counterflows of current causes the solvent, severely charged with monomers, to flow down from the upper zones of the extraction tube to the inlet of the fresh water. Because of these interference currents, the counterflow principle of the extraction and thereby the washing effect are seriously disturbed so that in the conventional extraction tubes, to sufficiently free the cuttings of the monomers, very large amounts of water are necessary. The maximum monomer concentration in the extraction tubes is about the middle so that the upper half of the tube remains virtually unused.

SUMMARYOF THE INVENTION It is therefore an important object of the invention to eliminate the disadvantages of prior art extraction tubes.

It is also an object of the invention to provide an extraction tube which is simple and economical in structure, yet highly efficient in use.

Further purposes and objects of the invention will appear as the specification proceeds.

It has been found that the objects of the invention may be accomplished by localization of secondary counterflows inside the extraction tube, whereby a more eflicient and more extensive removal of the lower molecular substances from the polyamide cuttings is achieved. This is accomplished by providing a plurality of installations over the height of the extraction tube for constricting the flow cross section of the extraction tube. The flow cross sections are chosen so that the upward flow of the solvent in the narrowest tube cross sections is equal to or greater than the counterflow of the solvent resulting from temperature variations and charging of the solvent with lower molecular components and/or a forced counterflow of the solvents.

These constrictions in the tube increase the flow rate of the solvent flowing in the tube. Experimentally, it has been found that the effect of improving the elutriation manifests itself effectively only if the narrowest of the flow cross sections are chosen so that the upward flow of the solvent in the constrictions is equal to or greater than the counterflow currents due to convection or density variations upon charging the solvent with lower molecular components. The constrictions in the tube act to block the downwardly directed interference currents of the solvent above the particular constriction in the tube.

A particular advantage of the invention is that the counterflow currents, which are actually desirable for intensifying the elutriation by providing good intermixing of granulate and solvent are not prevented, but rather are only localized. To provide further intensification of elutriation, additional streams created by suitable agitation or mixing assemblies may be utilized. In this case, the constrictions in the tube are dimensioned so that the upward movement of the solvent at the constrictions is equal to or greater than the counterflow of the solvent due to fluctuations in density and/or the forced counterflow of the solvent.

The size of constriction in the tube that is required for the desired blocking effect is readily calculated for each specific case from the mean flow velocity of the solvent and the flow brought about through density fluctuations of the solvent resulting from temperature variations and charging of the solvent with lower molecular components and/or a forced flow. The size of the constriction is liimted upon adversely affecting the down ward movement of the cuttings by reason of excessive solvent acceleration.

The extraction apparatus according to the invention has many advantages. In comparison to conventional equipment, and with equal granulate throughput and equal apparatus dimensions, to provide equal end monomer content in the granulate, much smaller amounts of solvent are required in the invention apparatus, for example, only about 50% to 70% of the amount normally required. In addition to the saving in solvent, there is also an improved working of the extraction agent for the recovery of the monomers because of the more concentrated collection thereof in water.

With equal amounts of water and like other conditions, the final concentration of the polyamide cuttinigs in caprolactam, using the apparatus of the invention, amounts to, for example, 0.3% by weight as compared to 1% to 1.5% by weight in the extraction in conventional apparatus. Furthermore, with the invention, a smaller extraction unit accomplishes the same final concentration in caprolactam as in a conventional extraction tower with the same amount of solvent. Further, it is important that the extraction apparatus according to the invention, particularly in the case of operational distrurbances, such as breakdown of the heating system and pressure loss in the solvent feed, has a more stable operating behavior than conventional exraction tubes.

BRIEF DESCRIPTION OF THE DRAWINGS Particular embodiments of the present invention are illustrated in the accompanying drawings, wherein:

FIG. 1 is a fragmentary, side elevational view of an extraction tube made according to the invention;

FIG. 2 is a fragmentary, side elevational view of another embodiment of the invention; and

FIG. 3 is a side elevational view of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an insert 2 being a funnel-shape shell truncated in the shape of a cone shell which narrows in the direction of movement of the granulate, which construction is advantageous in the case of a circular crosssection extraction tube 1. The narrowing of the fiow path is designated by the numeral 3. In the case of rectangular tubes, on the other hand, the installations are preferably in the shape of truncated pyramid shells. The direction of movement of the granulate is indicated by the arrow drawn in solid lines, while the arrow shown in broken lines shows the direction of flow of the solvent.

There are also other forms of construction, such as, for example, simple horizontal plates with corresponding restricted central apertures (not shown).

The restricted portions 3' may also, as shown in FIG. 2, be formed by funnel-shaped constrictions 2 defined in the outer wall of the extraction tube 1 itself, so that additional constricting inserts are unnecessary. In the funnelshaped constrictions 2 the use of the shown opposed double-funnel arrangements, with opposed aperture directions and meeting at the conical or pyramidal apexes of the funnel portions 2', are particularly advantageous since neither the flow of the solid matter nor that of the solvent is disturbed. In order to make a commercial scale extraction apparatus, for example, with funnel-shaped constrictions readily accessible for repairs and for the purpose of cleaning, short funnels with correspondingly large access openings, about the size of manholes, may be used.

In order to assure an adequate cross-sectional constriction, as shown in FIG. 3, relatively small conical caps 4 are positioned in the funnel apertures so that the apex of the cone of the cap 4 is directed opposite to the direction of movement of the granulate. With this construction, the constriction 3" is annular. These conical caps 4 are secured to suitable brackets (not shown) so that they can be swung out or removed. In this arrangement, operating personnel may pass through the funnel aperture for repair work. Such a construction of the extraction tube 1 for the extraction of poly-e-caprolactam is shown in FIG.

3. The extraction tube 1 contains funnel-shaped installations 2 of particularly restricted height and with corresponding removable conical caps 4, so that four annular flow constrictions 3 are provided. The extraction tube according to the invention can. be provided in a known manner with an outer insulating jacket and/or heating jacket (not shown). The operation of the extraction tube is best explained by describing a specific example of execution. Through a vibrating chute 5, about 15 kg. of granulate per hour was introduced into the top of the tube 1. The inlet temperature of the granulate was 40 C. and the concentration of the granulate in the caprolactam was 10% by weight. The granulate or cuttings remained in the tube 1 for about 20 hours. Through a pipe line 6 water was fed into the tube 1 at a temperature of about 110 C. The throughput of water was 15 kg./hr. An overflow 7 directed the water, charged with 10% by weight of caprolactam, out of the tube at a temperature of about C. The cuttings were discharged after completion of the extraction with the aid of a cell wheel sluice 8 and were dried in a conventional manner. The discharge temperature of the granulate was C. The final concentration of monomers in the granulate amounted to 0.5% by weight. At this concentration, threads with good properties can be readily made.

A conventional extraction tube without the constrictions of the invention yielded, with operating conditions otherwise the same, a granulate with a monomer content 1% to 1.5% by Weight. This amount of monomers is above the concentration required for the production of high quality threads.

While in the foregoing there has been provided a detailed description of various embodiments of the present invention, it is to be understood that all equivalents obvious to those having skill in the art are to be included within the scope of the invention as claimed.

I claim:

1. In a vertical extraction tube for removing lower molecular weight components from granulated polyarnides by solvents, including a solids inlet at the top of said tube, a solids outlet at the bottom of said tube, a solvent inlet in the bottom of said tube and a solvent outlet in the top of said tube, wherein the granulated polyamides [are introduced through said solids inlet into the upper end of said tube for downward fiow therethrough and the solvent is introduced through said solvent inlet into the lower end of said tube for upward flow therethrough countercurrent to the downward how of solids, the improvement comprising a plurality of vertically spaced flow constription means mounted in said tube and dividing said tube into plurality of vertically spaced stages, each of said stages providing a localization of solvent flow and counterfiow due to temperature variations and charging of said solvent with said lower molecular weight components, each of said flow constriction means consisting of a downwardly inwardly converging open bottomed frustoconical shell attached about its upper circumference to the sides of the tube and a small upwardly converging open bottomed conical cap mounted concentrically within said frustoconical shell, the apex of said cap being located substantially within the plane of the upper circumference of said frustoconical shell and the lower circumference of said cap being located in the plane substantially halfway between the upper and lower circumferences of said frustoconical shell, said constriction means defining a cross sectional area of said solvent flow therethrough governed by the spacing between the lower circumference of said cap and said frustoconical shell, said spacing having an upper dimensional limit and a lower dimensional limit, said upper limit defining a cross sectional area to provide a net solvent flow upwardly through said spacing, and said lower limit defining said cross sectional area to allow downward move-ment of granules through said spacing.

(References on following page) References Cited UNITED STATES PATENTS 6 OTHER REFERENCES Whatley et al.: O.R.N.L., 3533 (UC4-Chem., T.I.D., 4500), Oak Ridge National Laboratory, April 1964, pp.

8/1923 Johnston 23-283 X 12/1951 Mueller 23-270 5 1 FOREIGN PATENTS NORMAN YUDKOFF, Primary Examiner.

4/ 1950 Switzerland.

H1932 Italy. 5. I. EMERY, Asszstant Exammer.

6/1953 Germany. U.S CL X R 7/ 1963 Great Britain. 0 11/1962 Germany. 2331(); 19614.52; 260705 

